There has been generally known a system of condensing plural laser beams on a target at the same time to obtain a desired light intensity in a laser device required to output a significant power. In the laser device of such the type, in order to concentrate energy onto a fine target at a long distance, there has been studied such a technology that a laser beam that is produced by one main oscillator is split in plural laser beams, and then amplified individually, and the plural laser beams that have been amplified are so arranged as to bundle together (hereinafter, the plural laser beams that have been bundled together are called “main power beam”) to obtain a convergence performance that is equivalent to a single laser beam having a large diameter. Herein, such the technology is called “coherent coupling”. The prior art of a device that realizes the coherent coupling is disclosed in, for example, Patent Document 1.
In order to conduct the coherent coupling, it is necessary to control the phases of the respective laser beams so that the equivalent phase planes of the electromagnetic waves of the plural laser beams that constitute the main power beam are regarded as one wavefront. In such the control, there is required a phase out-of-synchronization detecting device that detects a relative phase difference of the plural laser beam. Patent Document 1 discloses a method in which a part of light is split, by means of a beam splitter, from the main oscillator which is a source of the plural laser beams as a reference light, and is interfered with the main power beam to detect a phase difference from the interference intensity.
Patent Document 1: JP 11-340555A (FIG. 5)